The present invention relates to an image readout element and, more particularly, to a close-contact type one-dimensional image readout sensor employing a photoconductive layer composed of photoconductive particles such as cadmium Selenide (CdSe) binded by a resin material is opto-electric transducer elements.
One-dimensional image sensors, for example, CCD image sensors, MOS image sensors, close-contact type image sensors, etc. have been developed for an image reading device for, for example, a facsimile machine, an intelligent copier or the like.
In one-dimensional solid-state image sensors such as MOS image sensors and CCD image sensors, an image on an original document is projected through slits and the projected image is de-magnified on the image sensor elements so as to output image information signals relating to the document image.
Both the MOS image sensor and the CCD image sensor are formed on a monocrystal semiconductor substrate through the use of the IC (Integrated Circuit) technique. Therefore, a large size sensor is difficult to form. Accordingly, the MOS image sensor and the CCD image sensor are generally combined with an optical lens system so as to form de-magnified images on the image sensor elements. The optical lens system requires a predetermined long light-path which prevents the reduction of the size of the image reading apparatus. Furthermore, control of the optical lens system in the image readout sensor is complex, in that light amount is decreased at a circumferential portions of the image screen and the resolution of the image is therefore reduced.
To solve the optical path problem, the close-constant type image sensor has been proposed, which includes a light responsive member having a longitudinal size greater than or equal to the original document size in width. An image of the same size as the original document is formed on the light responsive member via an optical fiber lens array. The close-contact type image sensor needs a light responsive member having the same longitudinal size as the original document width and, a uniform photoconductive layer of a considerably large size must be formed.
A solid solution layer consisting of a Cadmium Sulfide (CdS) chemical deposit layer an Cadmium Selenide (CdSe), an amorphous silicon hydride (a-Si:H) layer and a Se-As-Te type amorphous layer produced by plasma C.V.D. have been proposed so as to form the large size photoconductive layer. However, the close-contact type image readout sensor employing one of the above photoconductive layers has several serious problems which prevent the image readout sensor element from showing sufficient characteristics. For example, the image sensor employing the CdS-CdSe solid solution photoconductive layer has a complicated construction which complicates the manufacturing process therefor. Furthermore, the reading speed is limited by the light response speed of the photoconductive layer because the light response speed of the CdS-CdSe solid solution photoconductive layer is slow, and the range of the brightness ratio between "light" and "dark" is narrow. In another image sensor employing the a-Si:H photoconductive layer, the levels of opto-electric signals outputted from the a-Si:H photoconductive layer are low. Furthermore, the manufacturing cost of the image sensor is high and a specific machine for manufacturing the image sensor is needed and is expensive.